1. Field of the Invention
The present invention relates to magnesium alloy parts, and in particular to magnesium alloy parts having a paint film. The present invention also relates to a production method for such magnesium alloy parts.
2. Description of the Related Art
Conventionally, steels (i.e., iron alloys containing carbon) have frequently been used as component parts of transportation apparatuses because they excel in mechanical properties and ease of manufacturing while being inexpensive. However, the light-weight property of transportation apparatuses has been an issue in improving the mileage (fuel consumption) and running properties thereof, and use of materials that are lighter than iron has been under study.
In recent years, inexpensive refining methods for titanium, aluminum, magnesium, etc., which have smaller specific gravities than that of iron, and production methods for alloys containing such metals, have been developed. Moreover, techniques for improving the mechanical strength and ease of manufacturing of these metals and their alloys have also been developed.
Therefore, use of titanium, aluminum, or magnesium as a material of component parts of transportation apparatuses has been proposed. In particular, magnesium has a density which is about 23% of that of iron. Therefore, the weight of a transportation apparatus can be greatly reduced by using magnesium or a magnesium alloy.
In many cases, a paint film for protective or decorative purposes is provided on the surface of a magnesium alloy part. Electrostatic painting, electropainting, and the like are known methods for forming a paint film on the surface of a magnesium alloy. In particular, electropainting is able to uniformly form a paint film, and therefore is suitably used for parts with complex outer shapes.
Methods for forming a painting underlayer on a magnesium alloy are proposed in Japanese Laid-Open Patent Publication Nos. 2005-146329 and 2001-172772.
Japanese Laid-Open Patent Publication No. 2005-146329 discloses a technique of consecutively subjecting the surface of a magnesium alloy to an etching treatment with an organic acid and then a treatment with an aqueous solution of fluoride, followed by a conversion coating treatment with an aqueous solution of phosphate, thus reducing the electrical resistance and improving the painting anti-corrosiveness and paintability of the conversion coating.
Moreover, Japanese Laid-Open Patent Publication No. 2001-172772 discloses a technique of immersing a magnesium alloy in a treatment liquid containing a fluorochemical, to which a surface tension reducer is added, thus forming a conversion coating having an excellent anti-corrosiveness and smoothness on the surface of the magnesium alloy.
By using a conversion coating which is formed by either of the techniques of Japanese Laid-Open Patent Publication Nos. 2005-146329 and 2001-172772 as a painting underlayer, the durability of a paint film can be improved.
However, since transportation apparatuses are to be used mainly outdoor, their component parts are often subjected to harsh environments. This fact has led to the desire for a further improved adhesion (i.e., a stronger reluctance to peel) of a paint film. The inventors have conducted a humidity test for magnesium alloy parts each having a painting on a conversion coating which had been formed by either technique of Japanese Laid-Open Patent Publication Nos. 2005-146329 and 2001-172772. As a result, although good results were obtained under test conditions which are generally considered as “harsh”, peeling of the paint film was observed under harsher test conditions.